This invention is generally related to amplifiers and, in particular, to linear amplifiers.
Amplifiers are known to be either linear or nonlinear. Linearity is a measure of how closely the output signal of an amplifier resembles the input signal of the amplifier. Linear amplification is important when information is contained in the amplitude of a signal, such as amplitude modulated (AM) signals. Known digital wireless communication systems use a digital modulation scheme that impresses information on a radio frequency (RF) carrier signal by modulating both the amplitude and the phase of the RF carrier, such as a 16-ary quadrature amplitude modulation (16-ary QAM) digital modulation scheme. Since the amplitude of the RF carrier is being modulated, amplification is typically linear. In linear amplification, an RF amplifier, typically an RF power transistor, is typically biased for class A or class AB operation.
A drawback to class A and class AB bias operation is low efficiency. Efficiency is a measure of the level of conversion of input RF power and input DC power to output RF power. Class A bias amplifiers typically have efficiencies of well under 50% while class C (non-linear) bias amplifiers can approach 85% efficiency. The result of lower efficiency operation is greater power consumption to produce a desired level of output power, more limited RF output power for a given DC power source, and more complex thermal issues since an implication of lower efficiency is the dissipation of a greater percentage of the power consumed in the form of heat. These issues are critical when amplifier operation is constrained by battery life or when heat dissipation is constrained by transmitter size and an absence of fans.
To overcome the tradeoff of linearity for low efficiency, ideas have been developed in which linear operation is achieved while at the same time amplifiers are operated with a non-linear class C bias. One such linear amplifier is an envelope elimination and restoration (EER) amplifier, in which an RF signal is amplified by a class C biased amplifier and the RF signal""s amplitude is modulated by modulating the DC supply voltage of the amplifier. Problems faced by an open loop EER system is that they typically cannot achieve the stringent adjacent channel coupled power (ACCP) specifications of digital products such as the xe2x80x9ciDENxe2x80x9d product line of Motorola, Inc., of Schaumburg, Illinois or the TETRA (Terrestrial Trunked Radio) standards. Also, the bandwidth of the amplitude modulation provided by the modulation of the DC supply voltage is limited by the maximum switching rate of the DC power supply, which maximum switching rate with acceptable efficiency is generally about 1 Megahertz (MHz), with the result that the amplitude of the output signal is an incomplete replica of the amplitude of the input signal. And furthermore, variation of the supply voltage of the amplifier produces undesirable phase variation in the amplified signal.
To overcome the deficiencies of an open loop EER system, feedback loops have been added. Cartesian feedback loops require the use of quadrature amplitude modulators as a signal source for the RF amplifiers. However, quadrature modulators generate excessive wideband noise that must be filtered out by external filters. In the alternative, polar feedback loops have been proposed.
One polar feedback EER transmitter is described in a paper entitled xe2x80x9cEnvelope-elimination-and-restoration system concepts,xe2x80x9d by Frederick Raab, Proceedings of RF Expo East, Nov. 11-13, 1987, Boston, Mass., pp. 167-177. FIG. 1 is a block diagram illustration of Raab""s polar feedback EER transmitter 100, based on FIG. 5, p. 177, of Raab. Polar feedback transmitter 100 includes a top feedback path that provides amplitude (envelope) correction and a bottom feedback path that provides phase correction. The top feedback path includes two envelope detectors 104, 106, and the bottom feedback path includes two limiters 108, 110. Envelope detector 104 and limiter 108 are both coupled to an RF input node 102, and respectively generate amplitude and phase modulation signals at baseband frequencies based on the modulated RF input signal. There are a number of disadvantages to Raab""s transmitter 100. First, delays through the amplitude and phase paths must be very closely matched or off-channel intermodulation distortion (IMD) will result. Second, IMD is generated by non-linearities of the envelope detectors 104, 106. Even with the provision of matched envelope detectors, some mismatch in detection characteristics and the resultant IMD is inevitable. And third, the phase errors due to AM/PM conversion in the limiters 108, 110 is not corrected by the loop and will also result in IMD in the transmitter 100 output.
Another polar feedback loop transmitter is presented by Watkinson, U.S. Pat. No. 4,618,999. A block diagram illustration of Watkinson""s polar loop transmitter 200 is shown in FIG. 2. Watkinson eliminates the need for the matched envelope detectors required by Raab. However, as in Raab, Watkinson""s polar feedback loop resolves the loop error signal into phase and amplitude error signals by separate phase and amplitude feedback paths, which feedback paths compare the desired signal input with a sample of the output. As with Raab, this means that the IMD is sensitive to the matching of delays in the two feedback paths. Furthermore, the phase loop of Watkinson is nearly identical to the phase loop of Raab and, like Raab, requires the matching of two limiters (i.e., limiter 202 and limiter 204). As with Raab, the phase errors due to AM/PM conversion in the limiters 202, 204 is not corrected by the loop.
Therefore, a need exists for a method and apparatus for linear amplification of an RF signal, which method and apparatus provide the high efficiency possible with an EER amplifier, provide the ACCP and intermodulation distortion performance required for most digital communication systems, and are more tolerant of design and component variations than the prior art.